As the technology of implementing lithography with shorter wavelength light is established, the pattern can be processed to a ultrafine feature size. Since the depth of focus becomes shallower as the wavelength becomes shorter, synthetic quartz glass substrates for use in masks need completeness. Specifically, substrates for micropatterning reflective masks are desired to have a high flatness, low roughness, and mitigated defects on their surface.
It is also contemplated to use synthetic quartz glass substrates meeting the requirements of defect control and low roughness as the substrate for use in the micropatterning technology using the currently available ArF excimer laser (wavelength 193 nm), double-patterning or other shrinking mode, or EUV lithography.
Synthetic quartz glass substrates for use as photomasks, nanoimprint molds and liquid crystal filters must comply with the required specifications including high flatness, high smoothness, and defect control. To this end, they are subjected to several resurfacing steps including lapping and polishing steps before they are ready for use in the subsequent process. The lapping step is to remove work strains introduced by slicing from an ingot. The polishing step is to mirror finish the substrate for modifying the flatness and shape of its surface. The final polishing step is to polish the substrate with a polishing slurry of colloidal silica abrasive, obtaining a substrate with a flat and smooth surface and devoid of microscopic defects.
As the technique of finishing surfaces of synthetic quartz glass substrates to low defect density and low roughness, it is currently a common practice to carry out the final precision polishing step using a soft suede-type polishing pad along with a polishing slurry of colloidal silica abrasive.
Under the trend of EUV lithography toward the miniaturization of the design rule, active efforts are made to increase the polishing accuracy of the precision polishing step using colloidal silica slurry.
For example, Patent Document 1 discloses a method of polishing a glass substrate with a polishing slurry of silica grains under such conditions that the potential difference between the workpiece or glass substrate and agglomerates of silica grains may be up to 20 mV, for thereby preventing the silica agglomerates from depositing on the glass substrate. It is described that unlike the zeta potential of individual silica grains, the zeta potential of silica agglomerates has changed toward zero potential.
Although polishing with colloidal silica abrasive grains is effective for producing a substrate surface having a low roughness and low defectiveness, the polishing rate is outstandingly slow as compared with cerium oxide and zirconium oxide abrasive grains. While controlling the zeta potential so as to inhibit deposition of silica agglomerates onto the glass substrate surface is effective for surface modification, as described in Patent Document 1, there is a possibility that the polishing rate is further retarded from the polishing step using customary colloidal silica abrasive grains. From the industrial aspect, the process is not regarded as having satisfactory throughputs.